The present invention relates to a system and method of determining the remaining life of a centrifugal turbomachinery impeller. A centrifugal turbomachine may include one or more pump, turbine, or compressor impellers.
Centrifugal turbomachinery typically operate at high shaft speeds for best aerodynamic performance. At design speed the highest stresses approach yield strength of the materials typically used in this application, such as aluminum alloys. Generally, this can be accepted if the operating stress is steady, for example, fixed speed.
Turbomachinery equipment can be expected to operate either in a relatively steady mode at fixed speed or with variable speed. An example of a variable speed application is an air compressor that must produce a maximum pressure and then stop or return to idle mode at a lower speed to save energy. A typical idle speed is 30% of design speed where power is reduce to 3% of maximum power. The stresses in the impeller vary by the square of the speed.
When subjected to many start and stop cycles or random excursions in speed, the material can degrade and fail from fatigue. The life curve is a function of stress ratio, which is defined as the minimum stress divided by the maximum stress. Mean stress is the average of the maximum stress and the minimum stress. The amplitude for a given stress cycle is the maximum stress minus the minimum stress divided by two. The material strength also reduces with increasing temperature. If sufficient cycles are accumulated, the material cracks at the highest stress location and fails catastrophically due to the high mean stress from centrifugal loading. In practice, the speed can cycle from any minimum value to the maximum in a somewhat random nature depending upon the application. It is advantageous to predict with reasonable accuracy when the point of catastrophic failure may occur.